Controlled macroporous copolymer properties by removal of impurities in the diluent

ABSTRACT

Macroporous copolymer particles are prepared via a process wherein phase separating diluent is removed from copolymer particles and isolated (e.g., by distillation). The isolated diluent is purified by eluting the diluent through a column containing a purification agent such as alumina. The recycled diluent can be employed in the further preparation of macroporous copolymer particles.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of copending U.S. application Ser. No.07/161,271, filed Feb. 19, 1988, now abandoned, which was acontinuation-in-part of U.S. application Ser. No. 757,709, filed July22, 1985, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a process for manufacturing macroreticularcrosslinked copolymer particles, wherein the resulting copolymers aresubstantially consistent from batch to batch in pore size and porosity.These copolymer particles are capable of being converted into ionexchange resins.

Polymeric ion exchange resins in the form of spheroidal beads are usedin many applications as chelating resins and as anion or cationexchangers. Of particular interest are those resins prepared frommacroporous copolymer particles (i.e., copolymer particles having amacroreticular character). Such copolymer particles are typicallyprepared by suspending a suspension polymerizable monomer andcrosslinking agent in a reaction medium in the presence of a diluent inwhich the monomers are soluble, but the copolymer prepared therefrom isnot. During polymerization, the copolymers phase separate andagglomerate from the diluent phase. Thus, spaces or channels leftbetween the precipitated copolymer agglomerates form the pores of themacroporous particles. The diluent which can remain in the pores of thecopolymer particle is typically removed by techniques such as steamdistillation.

In commercial processes, after the macroporous copolymer particles arepolymerized and separated from the diluent, the diluent is recycled. Theproblem with the prior art is that the quality of the macroporouscopolymer particles varies a great deal. Therefore, it would be highlydesirable to provide a process for preparing copolymer particles havinga macroreticular character, which process is capable of producingcopolymer particles that exhibit high quality consistently.

Contrastingly from these commercial processes, we have discovered thatas recycled diluent is employed in the preparation of further amounts ofmacroporous copolymer particles, the diluent may become contaminated. Wehave also discovered that this change in diluent purity can result inthe preparation of macroporous copolymer particles which exhibitsignificant differences in particle porosity.

SUMMARY OF THE INVENTION

The present invention is an improvement process for manufacturingmacroreticular crosslinked copolymer particles capable of beingconverted into ion exchange resins, where the resulting copolymerparticles are substantially consistent from batch to batch in pore sizeand porosity.

The copolymer particles are prepared by polymerizing under suspensionpolymerization conditions a first monomer mixture of a monoethylenicallyunsaturated monomer and a polyethylenically unsaturated monomer, in thepresence of an inert diluent in which the monomer mixture is soluble andthe prepared copolymer particles are insoluble, thereby preparing afirst batch of macroreticular crosslinked copolymer particles and thenrecycling the diluent. However, prior to recycling the inert diluentfrom the first batch to a second batch monomer mixture, the inertdiluent is removed and purified, whereby the resulting macrorecticularcrosslinked copolymer particles produced in the second batch aresubstantially consistent in pore size and porosity with the first batch.By purifying the diluent, the copolymer particles have properties whichare consistent in numerous batches.

The process of this invention allows the skilled artisan to reemploydiluent numerous times in the preparation of macroporous copolymerparticles. Thus, the process provides for the more efficient use ofdiluent, which diluent is essentially free of species which adverselyaffect copolymer particle porosity. When such copolymer particles areconverted to ion exchange resins, such resins exhibit consistent waterretention capacity and ion exchange capacity.

DETAILED DESCRIPTION OF THE INVENTION

The monomers useful in the preparation of the copolymer particles areadvantageously suspension polymerizable ethylenically unsaturatedmonomers. Such suspension polymerizable monomers are well known in theart and reference is made to Copolymer Processes, edited by Calvin E.Schildknecht, published in 1956 by Interscience Publishers, Inc., NewYork, Chapter III, "Polymerization in Suspension" by E. Trommsdoff andC. E. Schildknecht, pp. 69-109 for purposes of illustration. In Table IIon pp. 78-81 of Schildknecht are listed diverse kinds of monomers whichcan be employed in the practice of this invention. Suitablemonoethylenically unsaturated monomers and polyethylenically unsaturatedmonomers useful in the practice of this invention are also disclosed inU.S. Pat. No. 4,419,245; which is incorporated herein by reference. Ofsuch suspension polymerizable monomers, of particular interest hereinare the water-insoluble monomers including the monovinylidene aromaticsuch as styrene, vinyl naphthalene, alkyl substituted styrenes(particularly monoalkyl substituted styrenes such as vinyltoluene andethyl vinylbenzene) and halo-substituted styrenes such as bromo- orchlorostyrene, the polyvinylidene aromatic such as divinylbenzene,divinyltoluene, divinyl xylene, divinyl naphthalene, trivinylbenzene,divinyl diphenyl ether, divinyl diphenyl sulfone and the like; esters ofα,β-ethylenically unsaturated carboxylic acids, particularly acrylic ormethacrylic acid, such as methyl methacrylate, ethyl acrylate, diversealkylene diacrylates and alkylene dimethacrylates, and mixtures of oneor more of said monomers. Of said monomers, the monovinylidenearomatics, particularly styrene or a mixture of styrene with a monoalkylsubstituted styrene; the polyvinylidene aromatics, particularlydivinylbenzene; esters of α,β-ethylenically unsaturated carboxylicacids, particularly methyl methacrylate or mixtures containing methylmethacrylate, particularly a mixture of styrene and divinylbenzene orstyrene, divinylbenzene and methyl methacrylate are preferably employedherein.

Preferably, the initiator, useful herein, is a conventional chemicalinitiator useful as a free radical generator in the polymerization ofethylenically unsaturated monomers. Representative of such initiatorsare UV radiation and chemical initiators including azo compounds such asazobisisobutyronitrile; peroxygen compounds such as benzoyl peroxide,t-butyl peroctoate, t-butyl perbenzoate and isopropylpercarbonate; andthe like. Several catalysts are disclosed in U.S. Pat. Nos. 4,192,921;4,246,386; and 4,283,499 and can be employed herein. The initiator isemployed in an amount sufficient to cause the copolymerization of themonomeric components in the monomer mixture. Such amount will generallyvary depending on a variety of factors including the type of initiatoremployed, and the type and proportion of monomers in the monomer mixtureused herein. Generally, the initiator is employed in amounts from about0.02 to about 1, preferably about 0.05, weight percent based on thetotal weight of the monomer mixture.

The monomers are advantageously suspended, using relatively highagitation rates, in a suitable suspending medium such as water or otheraqueous liquid. Suspending agents useful herein are those materialswhich assist in maintaining a more uniform dispersion of the droplets inthe aqueous liquid. Although the suspending agents most advantageouslyemployed herein are dependent on the type and amount of monomersemployed in preparing the droplets, in general, suspending agentsconventionally employed hereto in the suspension polymerization of mono-and polyethylenically unsaturated monomers are advantageously employed.Representative of such suspending agents are gelatin, polyvinyl alcohol,sodium methacrylate, magnesium silicate, sodium cellulose glycolate,hydroxyethylcellulose, methylcelluloses, carboxymethylmethylcellulose,and the like. Suitable suspending agents are disclosed in U.S. Pat. No.4,419,245.

While the amount of the suspending medium (e.g., aqueous phase)advantageously employed herein, will vary depending on the type andamount of the suspending agent and the amount of monomer, in general,the suspending medium is employed in amounts from about 30 to about 70,preferably from about 40 to about 60, weight percent based on the weightof the monomer mixture.

The copolymer particles can be prepared using a wide variety oftechniques known in the art. Conventional batch suspensionpolymerization techniques for preparing macroporous copolymer particlescan be employed. Also useful are those polymerization techniquesdescribed in U.S. Pat. No. 4,419,245.

Phase separating organic materials (i.e., diluent), which are necessaryherein, are preferably solvents for the monomer, but an agent which doesnot dissolve the copolymer. The phase separating organic material isinert in that it does not react with the monomers (with which it issoluble), the copolymers (with which it is insoluble), or the suspendingmedium. Generally, organic materials having boiling points greater thanabout 60° C. are employed. Suitable phase separating organic materialsinclude aromatic hydrocarbons, aliphatic hydrocarbons and halogenatedhydrocarbons. Examples include iso-octane, hexane, benzene, toluene,xylene, chlorobenzene, ethylene dichloride, ethylene dibromide, and thelike, as well as those similar materials disclosed in U.S. Pat. No.3,176,482. The amount of phase separating organic material which isemployed can vary and can range from about 10 to about 60 percent, basedon the weight of the organic material and monomer employed in theinitial suspension mixture. By the term "diluent" is meant phaseseparating organic materials.

By the term "functionally effective amount of diluent" is meant thattotal amount of fresh diluent plus recycled diluent necessary forfurther preparation of copolymer particles having a macroreticularcharacter. Typically, the added amount will be enough to provideessentially that total amount of diluent which was originally employedwhen consecutively preparing the same copolymer. However, that addedamount can vary if a different copolymer is prepared in a subsequent runwith the recycled diluent.

Polymerization conditions usually require agitation of the suspension ata rate which can vary depending upon factors such as the size of thereaction vessel while heating the mixture at a suitable temperature,preferably at about 75° to about 85° C. The polymerization conditionsare continued until polymerization of monomer is essentially complete.The reaction can be finished by raising the temperature of the reactionmixture.

The size of the copolymer particles or beads of this invention can varyand is advantageously in the range from about 50 to 2000 microns (μm),preferably from about 200 to 1200 μm. Control of the size of the beadsis achieved primarily by controlling the agitation rate, suspendingagent, and the amount of monomers employed.

The process of this invention comprises the removal of the diluent fromthe copolymer particles using techniques such as distillation, and inparticular, steam distillation. The diluent typically is removed andcontains various species which can hinder properties of productsprepared in further polymerization processes. In addition, the amount ofdiluent which is recovered is typically less than that amount which haspreviously been employed. Therefore, it is desirable to provide afurther amount of fresh diluent to the diluent which was previouslyemployed (i.e., the recycled diluent). However, impurities which arepresent in such a recycled diluent can be removed by subjecting therecycled diluent to a purification treatment.

Purification treatment comprises a treatment which removes impurities,such as reducible impurities, from the diluent. An example of apurification treatment involves contacting the diluent with at least apurifying amount of an agent such as alumina, silica gel, molecularsieves, and other such types of materials; particularly useful, arethose high surface area agents which can absorb and/or adsorb impuritiesfrom the diluent.

The amount and type of purifying agents depend upon a variety of factorssuch as the rate at which diluent is being treated and the amount andtypes of impurities which are present in the diluent. For example, acolumn containing about 18 grams of alumina at a height of 2 inches canbe used in treating about 7.5 liters of hydrocarbon diluent at a flowrate of 4 ml/minute. If desired, the column containing alumina can berecycled for further use by elution with methanol.

The copolymer particles or beads are converted to anion or cationexchange beads using techniques well known in the art for convertingcrosslinked addition copolymers of mono- and polyethylenicallyunsaturated monomer to such resins. See, for example, U.S. Pat. No.4,444,961.

The following example is given to further illustrate the invention andshould not be construed as limiting its scope. All parts and percentagesare given by weight unless otherwise indicated.

EXAMPLE 1

Into a stainless steel reactor are charged an aqueous phase and amonomer phase comprising a major amount of styrene, a monomer amount ofdivinylbenzene and a hydrocarbon diluent. The monomers are polymerizedusing a free-radical catalyst mixture in the presence of effectiveamounts of suspending agent and latex inhibitor at a temperature ofabout 80° C. for 7 hours and finishing at 110° C. for 5 hours. Agitatorrates during polymerization are 220 rpm in order to obtain copolymerparticles having a size range between 20 mesh and 60 mesh.

After polymerization is complete the copolymer beads are separated fromthe liquid phase by filtration and washed with water. The copolymerparticles are slurried in water in a two liter glass flask equipped withan agitator means, a thermometer in a vapor line, a condenser means anda receiving means. Steam is sparged into the bottom of the flask. Atabout 70° C. the hydrocarbon diluent evaporates off and is collectedafter condensation. The temperature is allowed to rise until most of thediluent is removed from the copolymer particles and collected. Thecopolymer particles are filtered and dried.

The collected hydrocarbon diluent is passed through a 12 inch by 0.5inch column of alumina at a flow rate of about 14 ml/minute.Polarographic analysis of the treated diluent indicates that essentiallyall of reducible species impurities are removed.

For illustration purposes the treated diluent is employed to preparemacroporous copolymer particles as described hereinbefore. The treatedbulk densities of the copolymer particles so prepared is 0.349 g/ml. Thetreated bulk density of copolymer particles which are similarly preparedbut using recycled but untreated diluent is 0.357 g/ml. The treated bulkdensity of copolymer particles prepared using fresh hydrocarbon diluentis about 0.344 g/ml. The treated bulk density values are determined byswelling the particles in toluene, exchanging the toluene withiso-octane in order to precipitate the copolymer in the swollen state.The iso-octane is then removed and the density of the dry copolymer isdetermined. The value is the treated bulk density.

To the treated diluent is added fresh hydrocarbon diluent in order toprovide the desired amount of diluent for further preparation ofmacroporous resin.

What is claimed is:
 1. In a process for recycling an inert diluent usedfor preparing macroreticular crosslinked copolymer particles, whereinthe copolymer particles are prepared by polymerizing under suspensionpolymerization conditions a first monomer mixture of a monoethylenicallyunsaturated monomer and a polyethylenically unsaturated monomer, in thepresence of an inert diluent in which the monomer mixture is soluble andthe prepared copolymer particles are insoluble, thereby preparing afirst batch of macroreticular crosslinked copolymer particles, and thenrecycling the inert diluent from the first batch to a second batchmonomer mixture, the improvement comprises purifying the inert diluentbefore the recycling step, whereby the resulting macrorecticularcrosslinked copolymer particles produced in each batch are substantiallyconsistent in pore size and porosity.
 2. The process of claim 1, whereinsaid purification treatment comprises contacting said diluent with atleast a purifying amount of alumina, silica or molecular sieves.
 3. Theprocess of claim 1, wherein said diluent is removed from said copolymerparticles and isolated using distillation techniques.
 4. The process ofclaim 1, wherein said diluent is a hydrocarbon.
 5. The process of claim1, wherein the monomers comprise monovinylidene aromatics,polyvinylidene aromatics and α,β-ethylenically unsaturated carboxylicacids.
 6. The process of claim 1, wherein the monomers are selected fromthe group consisting of styrene, divinylbenzene and methyl methacrylate.7. The process of claim 1, wherein an initiator is employed in thepolymerization of the monomers.
 8. The process of claim 7, wherein theinitiator is an azo compound.
 9. The process of claim 8, wherein theinitiator is azobisisobutyronitrile or peroxygen compound.
 10. Theprocess of claim 9, wherein peroxygen compound is benzoyl peroxide,t-butyl peroctoate, t-butyl perbenzoate or isopropylpercarbonate. 11.The process of claim 7, wherein the initiator is employed in amountsfrom about 0.02 to about 1 weight percent based upon total weight of themonomer mixture.
 12. The process of claim 11, wherein the initiator isemployed in an amount sufficient to yield about 0.05 weight percentbased upon total weight of the monomer mixture.